The present disclosure relates to processes for increasing the quality of printed images. It relates particularly to methods of increasing the resolution of halftone images and/or stabilizing such images during various transfer processes.
In the art of electrophotography, an imaging member or plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation, for example light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles, for example from a developer composition, on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving substrate such as paper. This imaging process may be repeated many times with reusable photoimaging members. This process is repeated multiple times for color images, which generally use multiple inks or toners of different color (e.g., cyan, magenta, yellow, black (CMYK)) to build up a final color image.
Imaging members are usually multilayered photoreceptors that comprise a supporting substrate, an electrically conductive layer, an optional hole-blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, and an optional protective or overcoat layer(s). For some multilayered flexible photoreceptor belts, an anti-curl layer is employed on the reverse side of the substrate support, opposite to the side carrying the electrically active layers, to achieve the desired photoreceptor flatness. They can be used in the form of photoreceptor drums or as flexible imaging member belts.
Halftoning is a known process of producing different scales of colors. Conceptually, halftones are produced by grouping arrays of pixels or dots together into a halftone cell. Within the cell, some or all of the pixels are printed. The scale depends on the number of pixels in the cell. For example, in an 8×8 cell using only black ink, a grey scale having 65 possible shades ranging from solid black (all pixels printed) to solid white (no pixels printed) is possible. With higher numbers of pixels in a cell, higher resolution is possible. When color images are used, scales can be produced for each ink color, and when combined, the total color palette available for printing can be very large. Halftoning is described in The Image Processing Handbook, second edition, 1995, by John C. Russ, ISBN 0-8493-2516-1, and Real World Scanning and Halftones, second edition, 1998, by David Blatner, Glenn Fleishman, and Stephen F. Roth, ISBN 0-201-69683-5. Both of these books are hereby incorporated by reference in their entirety.
However, one factor of the final image quality is whether the toner particles remain in the pixel in which they are deposited. The toner image developed on the surface of the photoconductive insulating layer can be disturbed during the transfer process onto paper. This is known to be one of the major contributors to increased image noise. For example, as toner particles spread out of their pixel, mottle and/or graininess increase. Mottle is a spotty or uneven appearance. Graininess is a sandpaper-like variation at higher spatial frequencies. Both result in a poorer image. In addition, the color palette of color images is influenced by the degree to which the pixels of each color are superimposed on each other. Again, as toner particles of a particular color spread out of their pixel, the final color seen by an observer will change.
It would be desirable to be able to print halftone images that have reduced image noise, i.e. increased image quality.